Why is flying to space so expensive?

The simple answer: Conservation of energy laws = ~You don't get something for nothing." We have to accelerate objects to 11.2 km/second to achieve escape velocity from the Earth, and every extra gram requires more energy to achieve this. Once you begin to throw in humanoids, the equation becomes incredibly difficult. You have to keep them alive - can't have too many G's, you have to keep them from freezing, being irradiated, they have to breathe, eat, drink, they produce waste, not to mention psychological issues (weak humans). Every single one of these accommodations requires equipment to do so, again adding valuable weight we must get off of the ground. This doesn't even approach the issues we'll encounter once we get to where we're going (landing without dying, setting up shop, etc.). The math is one thing, the raw materials/resources are a completely different matter altogether.